The present invention relates generally to error correction coding, and more particularly to a method and circuitry for error correction coding across multiple channels.
Traditional digital cable, satellite, and terrestrial distribution systems provide content streams (e.g., audio, video, data) over a large number of RF channels. Each RF channel may contain multiple content streams with each content stream typically delivered through a single RF channel. In conventional content distribution systems, each RF channel requires its own analog tuner. At the customer premises end, an end user selects a content stream via a remote control or other device. To select a particular content stream, an analog tuner is set to output only the RF channel containing the content stream of interest. A channel demodulator demodulates digital content from the tuner, and the content stream is then sent to an end user unit, which may a television, VCR, or computer.
FIG. 1 is a block diagram of a traditional content distribution system 100. The content distribution system 100 includes a content head end (CHE) 110 from which transmission originates, a medium 112 though which the transmission is sent, and customer premises equipment (CPE) 115 where the transmission is received. In a broadcast situation, more than one CPE 115 would be receiving the transmission. The CHE 110 includes n content stream blocks 118 which provide n streams of content to n corresponding RF channel blocks 120. The RF channel blocks 120 perform transmit functions such as modulation and produce n RF channels. The n RF channels form a multi-channel RF signal that is transferred through the medium 112.
Although the content distribution system 100 can include more than one CPE block 115, one CPE block 115 is expanded to show its major subsystems. The CPE 115 includes a tuner block 122 which receives one of the particular RF channels by selecting the RF channel and tuning to the RF frequency associated with that RF channel. The tuner block 122 provides the selected channel to a demodulation block 125 that demodulates the selected channel. The demodulation block 125 provides a demodulated channel signal to the end user 128 associated with the CPE 115.
A significant problem that plagues traditional content distribution systems is frequency-related interference. A myriad of possible sources may produce frequency-related interference, which can disrupt or completely preclude reception of particular RF channel(s). These sources can be internal to the system used to receive the desired signals. For example, these can be signal spectrum spurs generated by hardware within the system. These sources can also be external to the system and can thus change with the user's physical environment. Given the wide variety of possible sources of frequency-related interference in any real world environment, it may be extremely difficult to predict which RF channel will be disrupted and at what time the disruption will occur.
However, traditional content distribution systems, such as system 100, employ conventional in-channel error correction coding techniques to combat interference associated with transmission through the medium 112. Such techniques carry out error encoding and decoding for each channel independently of other channels. In the traditional content distribution system 100, in-channel encoding is performed in the RF channel blocks 120, and in-channel decoding is performed in the demodulation block 125. Accordingly, encoding performed in one RF channel block 120 is not correlated to encoding performed in another RF channel block 120. Similarly, decoding performed in the demodulation block 125 when a particular RF channel is selected does not correlate with decoding performed when another RF channel is selected.
Conventional in-channel error correction coding methods are only capable of correcting errors up to a certain level of signal degradation of the RF channel being received. If the conditions are so adverse that the RF channel is degraded beyond that level, the error correction method simply cannot recover the desired signal. This situation is played out when frequency-related interference precludes reception of a particular RF channel in a traditional content distribution system. All of the power of the interfering source is concentrated in one or more narrow bands of frequency. When a user decides to “tune in” to a particular RF channel that is sufficiently close to such a narrow frequency band of the interference, the concentrated power of the interfering source significantly impacts reception of the desired RF channel. The RF channel can easily be degraded beyond the correction ability of any reasonably designed in-channel error correction method implemented for the particular RF channel. Thus, the RF channel can be completely “knocked out.”
To compound the problem, any practical solution to frequency-related interference must address the difficult issue of backward compatibility with existing equipment. Many homes and businesses have already installed expensive equipment designed for traditional content distribution systems based on individual RF channels. Any new system that requires replacement or retooling of the immense quantity of such existing equipment currently in service is likely to waste resources and faces tremendous difficulty in being introduced into the market. Thus, the problem of frequency-related interference requires a solution that retains compatibility with equipment already in place based on the traditional structure of individual RF channels.